BAGHDAD -- Militant Sunnis from Iraq have been going to Syria to fight against President Bashar al-Assad for months. Now Iraqi Shiites are joining the battle in increasing numbers, but on the government's side, transplanting Iraq's explosive sectarian conflict to a civil war that is increasingly fueled by religious rivalry.

Some Iraqi Shiites are traveling to Tehran first, where the Iranian government, Syria's chief regional ally, is flying them to Damascus, Syria's capital. Others take tour buses from the Shiite holy city of Najaf, Iraq, on the pretext of making a pilgrimage to an important Shiite shrine in Damascus that for months has been protected by armed Iraqis. While the buses do carry pilgrims, Iraqi Shiite leaders say, they are also ferrying weapons, supplies and fighters to aid the Syrian government.

"Dozens of Iraqis are joining us, and our brigade is growing day by day," Ahmad al-Hassani, a 25-year-old Iraqi fighter, said by telephone from Damascus. He said that he arrived there two months ago, taking a flight from Tehran.

The Iraqi Shiites are joining forces with Shiite fighters from Lebanon and Iran, driving Syria ever closer to becoming a regional sectarian battlefield.

Lebanon, which has 100,000 Syrian refugees, was pushed to the brink this month when a Sunni intelligence chief was assassinated in a bombing. Many Lebanese blamed the Syrian government and its allies for the attack. Jordan, sheltering more than 180,000 refugees, has struggled to contain the violence on its border, which claimed the life of a Jordanian soldier in a firefight with extremists last week. Turkey, with more than 100,000 refugees, has traded artillery fire with Syria since Syrian shelling killed five civilians near the border early this month.

Now Iraq, still haunted by its own sectarian carnage, has become increasingly entangled in the Syrian war. And Iran, which, like Iraq, is majority-Shiite, appears to be playing a critical role in mobilizing Iraqis.

According to interviews with Shiite leaders here, the Iraqi volunteers are receiving weapons and supplies from the Syrian and Iranian governments, and Iran has organized travel for Iraqis willing to fight in Syria on the government's side.

Iran has also pressed the Iraqis to organize committees to recruit young fighters. Such committees have recently been formed in Iraq's Shiite heartland in the south and in Diyala Province, a mixed province north of Baghdad.

Many Iraqi Shiites increasingly see the Syrian war -- which pits the Sunni majority against a government dominated by Alawites, an offshoot of Shiite Islam -- as a battle for the future of Shiite faith. This sectarian cast has been heightened by the influx of Sunni extremists aligned with Al Qaeda, who have joined the fight against the Syrian government much as they did in the last decade against the Shiite-led Iraqi government.

"Syria is now open to all fighters, and Al Qaeda is playing on the chords of sectarianism, which will spur reactions from the Shiites, as happened in Iraq," said Ihsan al-Shammari, an analyst and professor at Baghdad University's College of Political Science. "My biggest fear from the Syrian crisis is the repercussions for Iraq, where the ashes of sectarian violence still exist."

One young Iraqi, Ali Hatem, who was planning to travel to Tehran, then to Damascus, said he saw the call to fight for Mr. Assad as part of a "divine duty."

Abu Mohamed, an official in Babil Province with the Sadrist Trend, a political party aligned with the populist Shiite cleric Moktada al-Sadr, said he recently received an invitation from the Sadrists' leadership to a meeting in Najaf to discuss a pilgrimage to the shrine of Sayyida Zeinab, a holy Shiite site in Damascus.

"We knew that this is not the real purpose because the situation is not suitable for such a visit," he said. "When we went to Najaf, they told us it's a call for fighting in Syria against the Salafis," ultraconservative Sunni Muslims.

A senior Sadrist official and former member of Parliament, speaking on condition of anonymity, said that convoys of buses from Najaf, ostensibly for pilgrims, were carrying weapons and fighters to Damascus.

Iran, which has been accused of sending weapons and fighters to Syria, may have employed the same ruse. After the Syrian rebels detained 48 Iranians in Damascus in August, the Iranian government said they were pilgrims, and expressed outrage that they had been kidnapped by the rebels. According to American intelligence officials, at least some of the pilgrims were members of Iran's elite Islamic Revolutionary Guards Corps.

Religious warriors, however, do not always make such distinctions. In Diyala Province, still a hotbed of Iraq's Sunni insurgency, Shiite leaders say they are seeking volunteers for a "combat regiment" to defend the Zeinab shrine against "the holders of extremist Salafi ideology backed by gulf states," according to Abu Ali al-Moussawi, the head of a recruitment committee. He said that 70 men from Diyala had recently left to join the fight in Syria.

Abu Sajad, who moved to Damascus in 2008 and joined the fight after the rebellion began, said he and other Iraqi fighters were indeed fighting to protect the shrine. A former fighter in Mr. Sadr's Mahdi Army in Iraq, he said he was given weapons and supplies by the Syrian government.

But as the fight evolved, and Iraqis began to be killed and kidnapped, it reminded him too much of the Iraq he left, and so he recently returned to his home in Basra.

"I can tell that things are going to be crazy in Syria," he said. "It's a sectarian war, and it's even worse than the one we had here, which was between the militias and the political parties. In Syria, all of the people are involved. You can feel the hatred between the Sunnis and the Alawites. They will do anything to get rid of each other."

Iraqi Shiites did not initially take sides in Syria. Many Shiites here despise Mr. Assad for his affiliation with the Baath Party, the party of Saddam Hussein, and the support he gave foreign Sunni fighters during the Iraq war.

But as the uprising became an armed rebellion that began to attract Sunni extremists, many Shiites came to see the war in existential terms. Devout Shiites in Iraq often describe the Syrian conflict as the beginning of the fulfillment of a Shiite prophecy that presages the end of time by predicting that an army, headed by a devil-like figure named Sufyani, will rise in Syria and then conquer Iraq's Shiites.

It was the bombing of an important shrine in Samarra in 2006 that escalated Iraq's sectarian civil war, and many Iraqis see the events in Syria as replicating their own recent bloody history, but with even greater potential consequences.

Hassan al-Rubaie, a Shiite cleric from Baquba, the capital of Diyala Province, said, "The destruction of the shrine of Sayyida Zeinab in Syria will mean the start of sectarian civil war in Iraq, Syria, Lebanon, Bahrain and Saudi Arabia."

Yasir Ghazi reported from Baghdad and Tim Arango from Istanbul. Employees of The New York Times contributed reporting from Babil and Diyala Provinces in Iraq.